LATE PLEISTOCENE ICE-RAFTED DEBRIS FLUXES FROM THE BERING GLACIER, ALASKA

The Bering Glacier, the largest in North America, sits adjacent to the North Pacific and responds greatly to modern regional and global climate changes. Yet, the glacier’s response to late Quaternary climate change is uncertain. The ice-rafted debris (IRD >250 mm) flux rate and mass accumulation rate from 10-18 cal kyr BP were studied from a jumbo piston core (EW0408 85JC; 682 m water depth) off the Bering Glacier to document the response to climate during this period. Grain size analysis was performed to isolate the IRD fraction. A 34-point planktic foraminiferal C-14 age model and shipboard gamma-ray attenuation bulk density data were used to calculate the total mass and IRD flux rates.

From ~17.4 to 16.6 cal kyr BP, the IRD content is high (2-12% by mass), and the bulk sediment and IRD flux rate decrease exponentially from 4 to 0.02 g/cm²/yr and 150 to 0.3 mg/cm²/yr, respectively. Between 16.6 and 14.9 cal kyr BP, the bulk sediment flux remains relatively low and steady at 0.02 g/cm²/yr. However, the IRD content increases exponentially from a low of 1.5% peaking at 11.5 % at 15.2 cal kyr BP, and the IRD flux follows a similar pattern. Both the content and flux drastically decrease to zero at 14.8 cal kyr BP. Between 10.0 and 14.8 cal kyr BP, the bulk sediment flux varies around ~0.4 g/cm²/yr, but only a single sample at 12.6 cal kyr BP contained measurable IRD.

Seismic reflection and bathymetric data and the high IRD and bulk sediment fluxes prior to 17.0 cal kyr BP indicate that the Bering was likely at tidewater at this time. The exponentially decreasing fluxes between 16.6-17.4 cal kyr BP indicate that the terminus of the glacier retreated landward. The final IRD pulse and relatively low sediment fluxes between 14.9-16.6 cal kyr BP, followed by no IRD and low sediment fluxes would indicate a final period of intensified calving that lead to the Bering no longer being tidewater. The period of high flux rates are within the Heinrich 1 event (H1) time period. The retreat could have resulted from regional atmospheric warming and sea-level rise, which were suggested as the cause for Cordilleran Ice Sheet retreat in British Columbia (Hendy and Cosma, 2008). The absence of IRD after 14.8 cal kyr BP corresponds with the transition into the Bølling-Allerød, which suggests that the B-A coincides with substantial changes to glacial mass balances in coastal Alaska.